Omega OM-USB-TEMP-AI User manual

omega.com
e-mail: [email protected]
For latest product manuals:
omegamanual.info
OM-USB-TEMP-AI
8 Channel
Temperature/Voltage Measurement
USB Data Acquisition Module
Shop online at
User’s Guide
Extended Warranty
Program
SM

Servicing North America:
U.S.A.: Omega Engineering, Inc., One Omega Drive, P.O. Box 4047
ISO 9001 Certified Stamford, CT 06907-0047 USA
Toll Free: 1-800-826-6342 TEL: [phone]
FAX: [phone] e-mail: [email protected]
Canada: 976 Bergar
Laval (Quebec), H7L 5A1 Canada
Toll-Free: 1-800-826-6342 TEL: [phone]
FAX: [phone] e-mail: [email protected]
For immediate technical or application assistance:
U.S.A. and Canada: Sales Service: 1-800-826-6342/1-800-TC-OMEGA®
Customer Service: 1-800-622-2378/1-800-622-BEST®
Engineering Service: 1-800-872-9436/1-800-USA-WHEN®
Mexico/ En Español: 001 [phone] FAX: [phone]
Latin America: [email protected] e-mail: [email protected]
Servicing Europe:
Benelux: Managed by the United Kingdom Office
Toll-Free: 0800 099 3344 TEL: [phone]
FAX: [phone] e-mail: [email protected]
Czech Republic: Frystatska 184
733 01 Karviná, Czech Republic
Toll-Free: 0800-1-66342 TEL: [phone]
FAX: [phone] e-mail: [email protected]
France: Managed by the United Kingdom Office
Toll-Free: 0800 466 342 TEL: [phone]
FAX: [phone] e-mail: [email protected]
Germany/Austria: Daimlerstrasse 26
D-75392 Deckenpfronn, Germany
Toll-Free: 0800 6397678 TEL: [phone]
FAX: [phone] e-mail: [email protected]
United Kingdom: OMEGA Engineering Ltd.
ISO 9001 Certified One Omega Drive, River Bend Technology Centre, Northbank
Irlam, Manchester M44 5BD United Kingdom
Toll-Free: 0800-488-488 TEL: [phone]
FAX: [phone] e-mail: [email protected]
OMEGAnet®Online Service Internet e-mail
omega.com [email protected]
It is the policy of OMEGA Engineering, Inc. to comply with all worldwide safety and EMC/EMI
regulations that apply. OMEGA is constantly pursuing certification of its products to the European New
Approach Directives. OMEGA will add the CE mark to every appropriate device upon certification.
The information contained in this document is believed to be correct, but OMEGA accepts no liability for any
errors it contains, and reserves the right to alter specifications without notice.
WARNING: These products are not designed for use in, and should not be used for, human applications.


Table of Contents
Preface
About this User’s Guide............................................................................................................................. 5
What you will learn from this user’s guide.............................................................................................................. 5
Conventions in this user’s guide............................................................................................................................... 5
Where to find more information............................................................................................................................... 5
Chapter 1
Introducing the OM-USB-TEMP-AI...........................................................................................................6
Overview: OM-USB-TEMP-AI features.................................................................................................................. 6
OM-USB-TEMP-AI block diagram ......................................................................................................................... 7
Software features....................................................................................................................................................... 7
Connecting a OM-USB-TEMP-AI to your computer is easy.................................................................................. 8
Chapter 2
Installing the OM-USB-TEMP-AI ...............................................................................................................9
What comes with your OM-USB-TEMP-AI shipment?.......................................................................................... 9
Hardware.....................................................................................................................................................................................9
Additional documentation..........................................................................................................................................................9
Unpacking the OM-USB-TEMP-AI......................................................................................................................... 9
Installing the software............................................................................................................................................. 10
Installing the OM-USB-TEMP-AI ......................................................................................................................... 10
Configuring the OM-USB-TEMP-AI..................................................................................................................... 10
Calibrating the OM-USB-TEMP-AI ...................................................................................................................... 10
Chapter 3
Signal I/O Connections ............................................................................................................................11
Screw terminal pin out ............................................................................................................................................ 11
Voltage input terminals (±V0H/V0L to ±V3H/V3L) .............................................................................................................12
Sensor input terminals (T0H/T0L to T3H/T3L)......................................................................................................................12
Current excitation output terminals (±I1 and ±I2) ..................................................................................................................13
Four-wire, two sensor common terminals (4W01 and 4W23) ...............................................................................................13
Sensor common terminals (IT01 and IT23).............................................................................................................................13
Digital terminals (DIO0 to DIO7)............................................................................................................................................13
Counter terminal (CTR) ...........................................................................................................................................................13
CJC sensor.................................................................................................................................................................................13
Ground terminals (GND)..........................................................................................................................................................13
Power terminal (+5V)...............................................................................................................................................................14
Thermocouple connections ..................................................................................................................................... 14
Wiring configuration ................................................................................................................................................................14
RTD and thermistor connections............................................................................................................................ 14
Two-wire configuration............................................................................................................................................................15
Three-wire configuration..........................................................................................................................................................16
Four-wire configuration............................................................................................................................................................16
Semiconductor sensor measurements..................................................................................................................... 17
Wiring configuration ................................................................................................................................................................18
Digital I/O connections ........................................................................................................................................... 18
Chapter 4
Functional Details.....................................................................................................................................19
Thermocouple measurements ................................................................................................................................. 19
Cold junction compensation (CJC)..........................................................................................................................................19
Data linearization......................................................................................................................................................................19

OM-USB-TEMP-AI User's Guide
4
Open-thermocouple detection (OTD)......................................................................................................................................19
RTD and thermistor measurements ........................................................................................................................ 19
Data linearization......................................................................................................................................................................20
USB connector......................................................................................................................................................... 20
LED.......................................................................................................................................................................... 20
Power ....................................................................................................................................................................... 20
Chapter 5
Specifications............................................................................................................................................21
Analog input ............................................................................................................................................................ 21
Channel configurations ........................................................................................................................................... 23
Compatible sensors: T0x-T3x..................................................................................................................................................24
Accuracy.................................................................................................................................................................. 24
Thermocouple measurement accuracy: T0x-T3x....................................................................................................................24
Semiconductor sensor measurement accuracy: T0x-T3x .......................................................................................................25
RTD measurement accuracy: T0x-T3x....................................................................................................................................25
Thermistor measurement accuracy: T0x-T3x..........................................................................................................................26
Absolute Accuracy: V0x-V3x..................................................................................................................................................27
Settling time: V0x-V3x ............................................................................................................................................................28
Analog input calibration.......................................................................................................................................... 28
Throughput rate ....................................................................................................................................................... 28
Digital input/output ................................................................................................................................................. 29
Counter..................................................................................................................................................................... 29
Memory.................................................................................................................................................................... 30
Microcontroller........................................................................................................................................................ 30
USB +5V voltage .................................................................................................................................................... 30
Power ....................................................................................................................................................................... 30
USB specifications .................................................................................................................................................. 30
Current excitation outputs (±Ix, T0x-T3x)............................................................................................................. 31
Environmental ......................................................................................................................................................... 31
Mechanical............................................................................................................................................................... 31
Screw terminal connector type and pin out............................................................................................................ 32
Screw terminal pin out..............................................................................................................................................................32

5
Preface
About this User’s Guide
What you will learn from this user’s guide
This user’s guide explains how to install, configure, and use the OM-USB-TEMP-AI so that you get the most
out of its USB-based temperature and voltage measurement features.
This user’s guide also refers you to related documents available on our web site, and to technical support
resources.
Conventions in this user’s guide
For more information on …
Text presented in a box signifies additional information and helpful hints related to the subject matter you are
reading.
Caution! Shaded caution statements present information to help you avoid injuring yourself and others,
damaging your hardware, or losing your data.
<#:#> Angle brackets that enclose numbers separated by a colon signify a range of numbers, such as those assigned
to registers, bit settings, etc.
bold text Bold text is used for the names of objects on the screen, such as buttons, text boxes, and check boxes. For
example:
1. Insert the disk or CD and click the OK button.
italic text Italic text is used for the names of manuals and help topic titles, and to emphasize a word or phrase. For
example:
Never touch the exposed pins or circuit connections on the board.
Where to find more information
For additional information relevant to the operation of your hardware, refer to the Documents subdirectory
where you installed the software, or search for your device on our website at www.omega.com.

6
Chapter 1
Introducing the OM-USB-TEMP-AI
Overview: OM-USB-TEMP-AI features
This user's guide contains all of the information you need to connect the OM-USB-TEMP-AI to your computer
and to the signals you want to measure.
The OM-USB-TEMP-AI is a USB 2.0 full-speed, temperature measurement module that is supported under
popular Microsoft®Windows®operating systems. The OM-USB-TEMP-AI is fully compatible with both USB
1.1 and USB 2.0 ports.
The OM-USB-TEMP-AI provides eight analog input channels that are configured as four differential
temperature inputs and four differential or single-ended voltage inputs. A 24-bit analog-to-digital (A/D)
converter is provided for each pair of analog inputs. Eight independent, TTL-compatible digital I/O channels
are provided to monitor TTL-level inputs, communicate with external devices, and to generate alarms. The
digital I/O channels are software programmable for input or output.
The temperature input channels are configured as two channel pairs that accept temperature sensor type inputs.
You can take measurements from four sensor categories. The sensor category is software programmable for
each channel pair:
Thermocouple – types J, K, R, S, T, N, E, and B
Resistance temperature detectors (RTDs) – 2, 3, or 4-wire measurements of 100 _ platinum RTDs
Thermistors – 2, 3, or 4-wire measurements
Semiconductor temperature sensors – LM36 or equivalent
You can connect a different category of sensor to each temperature channel pair, but you cannot mix sensor
categories between the channels that constitute a channel pair. You can, however, mix thermocouple types
within channel pairs.
Each voltage input channel is software configurable for differential or single-ended mode. The voltage input
range is software programmable for ±10 V, ±5 V, ±2.5 V, ±1.25 V.
The OM-USB-TEMP-AI provides a integrated cold junction compensation (CJC) sensor for thermocouple
measurements, and built-in current excitation sources for resistive sensor measurements.
An open thermocouple detection feature lets you detect a broken thermocouple. An on-board microprocessor
automatically linearizes the measurement data according to the sensor category.
The OM-USB-TEMP-AI is a standalone plug-and-play module which draws power from the USB cable. No
external power supply is required. All configurable options are software programmable.
The OM-USB-TEMP-AI is fully software calibrated.

OM-USB-TEMP-AI User's Guide Introducing the OM-USB-TEMP-AI
7
OM-USB-TEMP-AI block diagram
OM-USB-TEMP-AI functions are illustrated in the block diagram shown here.
Figure 1. OM-USB-TEMP-AI functional block diagram
Software features
For information on the features of InstaCal and the other software included with your OM-USB-TC, refer to the
OMB-DAQ-2416 Series and OM-USB Series Data Acquisition Software User’s Guide that shipped with your
device.

OM-USB-TEMP-AI User's Guide Introducing the OM-USB-TEMP-AI
8
Connecting a OM-USB-TEMP-AI to your computer is easy
Installing a data acquisition device has never been easier:
The OM-USB-TEMP-AI relies upon the Microsoft Human Interface Device (HID) class drivers. The HID
class drivers ship with every copy of Windows that is designed to work with USB ports. We use the
Microsoft HID because it is a standard, and its performance delivers full control and maximizes data
transfer rates for your OM-USB-TEMP-AI. No third-party device driver is required.
The OM-USB-TEMP-AI is plug-and-play. There are no jumpers to position, DIP switches to set, or
interrupts to configure.
You can connect the OM-USB-TEMP-AI before or after you install the software, and without powering
down your computer first. When you connect an HID to your system, your computer automatically detects
it and configures the necessary software. You can connect and power multiple HID peripherals to your
system using a USB hub.
You can connect your system to various devices using a standard USB cable. The USB connector replaces
the serial and parallel port connectors with one standardized plug and port combination.
You do not need a separate power supply module. The USB automatically delivers the electrical power
required by each peripheral connected to your system.
Data can flow two ways between a computer and peripheral over USB connections.

9
Chapter 2
Installing the OM-USB-TEMP-AI
What comes with your OM-USB-TEMP-AI shipment?
The following items are shipped with the OM-USB-TEMP-AI.
Hardware
OM-USB-TEMP-AI
USB cable (2 meter length)
Additional documentation
In addition to this hardware user's guide, you should also receive the OMB-DAQ-2416 Series and OM-USB
Series Data Acquisition Software User’s Guide. This booklet supplies a brief description of the software you
received with your OM-USB-TEMP-AI and information regarding installation of that software. Please read this
booklet completely before installing any software or hardware.
Unpacking the OM-USB-TEMP-AI
As with any electronic device, you should take care while handling to avoid damage from static
electricity. Before removing the OM-USB-TEMP-AI from its packaging, ground yourself using a wrist strap or
by simply touching the computer chassis or other grounded object to eliminate any stored static charge.
If any components are missing or damaged, notify Omega Engineering immediately by phone, fax, or e-mail.
Phone: [phone]
Fax: [phone]
Email: [email protected]

OM-USB-TEMP-AI User's Guide Installing the OM-USB-TEMP-AI
10
Installing the software
Refer to the OMB-DAQ-2416 Series and OM-USB Series Data Acquisition Software User’s Guide for
instructions on installing the software on the OMB-DAQ-2416 Series and OM-USB Series Data Acquisition
Software CD. This booklet is available in PDF at http://omega.com/manuals.
We recommend that you download the latest Windows Update onto your computer before installing and
operating the OM-USB-TEMP-AI.
Installing the OM-USB-TEMP-AI
To connect the OM-USB-TEMP-AI to your system, turn your computer on, and connect the USB cable to a
USB port on your computer or to an external USB hub that is connected to your computer. The USB cable
provides power and communication to the OM-USB-TEMP-AI.
When you connect the OM-USB-TEMP-AI for the first time, a notification message opens as the OM-USB-
TEMP-AI is detected. When the message closes, the installation is complete. The USB LED should flash and
then remain lit. This indicates that communication is established between the OM-USB-TEMP-AI and your
computer.
If the LED turns off
If the LED is lit but then turns off, the computer has lost communication with the OM-USB-TEMP-AI. To
restore communication, disconnect the USB cable from the computer, and then reconnect it. This should restore
communication, and the LED should turn back on.
Configuring the OM-USB-TEMP-AI
All hardware configuration options on the OM-USB-TEMP-AI are programmable with software. Use InstaCal
to set the sensor type for each temperature channel and the range and input configuration of each voltage
channel. Any channel you don’t intend to use should be left disabled.
The configurable options dynamically update according to the selected sensor category. Configuration options
are stored on the OM-USB-TEMP-AI 's isolated microcontroller in EEPROM, which is non-volatile memory on
the OM-USB-TEMP-AI module. Configuration options are loaded on power up.
Default configuration
The factory default configuration is Disabled. The Disabled mode disconnects the analog inputs from the
terminal blocks and internally grounds all of the A/D inputs. This mode also disables each of the current
excitation sources.
Warm up
Allow the OM-USB-TEMP-AI to warm up for 30 minutes before taking measurements. This warm up time
minimizes thermal drift and achieves the specified rated accuracy of measurements.
For analog, RTD or thermistor measurements, this warm-up time is also required to stabilize the internal current
reference.
Calibrating the OM-USB-TEMP-AI
The OM-USB-TEMP-AI is fully calibrated via software. InstaCal prompts you to run its calibration utility
when you change from one sensor category to another.
Allow the OM-USB-TEMP-AI to operate for at least 30 minutes before calibrating. This warm up time
minimizes thermal drift and achieves the specified rated accuracy of measurements.


Chapter 3
Signal I/O Connections
Screw terminal pin out
The OM-USB-TEMP-AI has four rows of screw terminals — two rows on the top edge of the housing, and two
rows on the bottom edge. Each row has 26!connections. Between screw terminals 10 and 11 is the integrated
CJC sensor used for thermocouple measurements. Signals are identified in Figure 2.
Figure 2. OM-USB-TEMP-AI screw terminal pin numbers

OM-USB-TEMP-AI User's Guide Signal I/O Connections
12
OM-USB-TEMP-AI screw terminal descriptions
Pin Signal Name Pin Description Pin Signal Name Pin Description
1 I1+ T0/T1 current excitation source 27 GND
2 NC 28 V3L V3 voltage input (-)
3 T0H T0 sensor input (+) 29 V3H V3 voltage input (+)
4 T0L T0 sensor input (-) 30 GND
5 4W01 T0/T1 4-wire, 2 sensor common 31 V2L V2 voltage input (-)
6 IT01 T0/T1 2-sensor common 32 V2H V2 voltage input (+)
7 T1H T1 sensor input (+) 33 GND
8 T1L T1 sensor input (-) 34 V1L V1 voltage input (-)
9 GND 35 V1H V1 voltage input (+)
10 I1- T0/T1 current excitation return 36 GND
CJC sensor
11 I2+ T2/T3 current excitation source 37 V0L V0 voltage input (-)
12 NC 38 V0H V0 voltage input (+)
13 T2H T2 sensor input (+) 39 GND
14 T2L T2 sensor input (-) 40 CTR Counter Input
15 4W23 T2/T3 4-wire, 2 sensor common 41 DIO7 Digital Input/Output
16 IT23 T2/T3 2 sensor common 42 DIO6 Digital Input/Output
17 T3H T3 sensor input (+) 43 DIO5 Digital Input/Output
18 T3L T3 sensor input (-) 44 DIO4 Digital Input/Output
19 GND 45 DIO3 Digital Input/Output
20 I2- T2/T3 current excitation return 46 DIO2 Digital Input/Output
21 +5V +5V output 47 DIO1 Digital Input/Output
22 GND 48 DIO0 Digital Input/Output
23 NC 49 GND
24 NC 50 NC
25 NC 51 NC
26 NC 52 NC
Use 16 AWG to 30 AWG wire for your signal connections.
Tighten screw terminal connections
When making connections to the screw terminals, be sure to tighten the screw until tight. Simply touching the
top of the screw terminal is not sufficient to make a proper connection.
Voltage input terminals (±V0H/V0L to ±V3H/V3L)
You can connect up to four voltage inputs to the voltage channels (V0H/V0L to V3H/V3L). The input range is
software programmable for ±10 V, ±5 V, ±2.5 V, or ±1.25 V. Each voltage channel is software configurable for
differential or single-ended mode.
When connecting differential inputs to floating input sources, you must provide a DC return path from each
differential input to ground. One way to do this is to connect a resistor from one side of each of the differential
inputs to GND. A value of approximately 100 k_ can be used for most applications.
Caution! All ground pins on the OM-USB-TEMP-AI (pins 9, 19, 22, 27, 30, 33, 36, 39, 49) are common
and are isolated from earth ground. If a connection is made to earth ground when using digital I/O
and conductive thermocouples, the thermocouples are no longer isolated. In this case,
thermocouples must not be connected to any conductive surfaces that may be referenced to earth
ground.
Sensor input terminals (T0H/T0L to T3H/T3L)
The OM-USB-TEMP-AI supports the following temperature sensor types:
Thermocouple – types J, K, R, S, T, N, E, and B
Resistance temperature detectors (RTDs) – 2, 3, or 4-wire measurement modes of 100 _ platinum RTDs.
Thermistors – 2, 3, or 4-wire measurement modes.
Semiconductor temperature sensors – LM36 or equivalent

OM-USB-TEMP-AI User's Guide Signal I/O Connections
13
Sensor selection
The type of sensor you select will depend on your application needs. Review the temperature ranges and
accuracies of each sensor type to determine which is best suited for your application.
You can connect up to four temperature sensors to the differential sensor inputs (T0H/T0L to T3H/T3L).
Supported sensor categories include thermocouples, RTDs, thermistors, or semiconductor sensors.
Do not mix sensor categories within channel pairs. You can mix thermocouple types (J, K, R, S, T, N, E, and B)
within channel pairs, however.
Do not connect two different sensor categories to the same channel pair
The OM-USB-TEMP-AI provides a 24 bit A/D converter for each channel pair. Each channel pair can monitor
one sensor category. To monitor a sensor from a different category, connect the sensor to a different pair of
sensor input terminals.
Current excitation output terminals (±I1 and ±I2)
The OM-USB-TEMP-AI has two dedicated pairs of current excitation output terminals (±I1 and ±I2). These
terminals have a built-in precision current source to provide excitation for the resistive sensors used for RTD
and thermistor measurements. Each current excitation terminal is dedicated to one pair of sensor input channels:
I1+ is the current excitation source, and I1- is the current excitation return for channel 0 and channel 1
I2+ is the current excitation source, and I2- is the current excitation return for channel 2 and channel 3
Four-wire, two sensor common terminals (4W01 and 4W23)
The 4W01 and 4W23 terminals are used as the common connection for four-wire configurations with two RTD
or thermistor sensors.
Sensor common terminals (IT01 and IT23)
The IT01 and IT23 terminals are used as the common connection for two-wire configurations with two RTD or
thermistor sensors.
Digital terminals (DIO0 to DIO7)
You can connect up to eight digital I/O lines to the screw terminals labeled DIO0 to DIO7. Each terminal is
software configurable for input or output.
Counter terminal (CTR)
The CTR terminal (pin 40) is the input to the 32-bit event counter. The internal counter increments when the
TTL level transitions from low to high. The counter can count events at frequencies of up to 1 MHz.
Caution! All ground pins on the OM-USB-TEMP-AI (pins 9, 19, 22, 27, 30, 33, 36, 39, 49) are common
and are isolated from earth ground. If a connection is made to earth ground when using digital I/O
and conductive thermocouples, the thermocouples are no longer isolated. In this case,
thermocouples must not be connected to any conductive surfaces that may be referenced to
earth ground.
CJC sensor
The OM-USB-TEMP-AI has one built-in high-resolution temperature sensor. The CJC sensor measures the
ambient temperature at the terminal block so that the cold junction voltage can be calculated.
Ground terminals (GND)
The nine ground terminals (GND) provide a common ground for the input channels and DIO bits and are
isolated (500 VDC) from the USB GND.

OM-USB-TEMP-AI User's Guide Signal I/O Connections
14
Power terminal (+5V)
The +5V output terminal is isolated (500 VDC) from the USB +5V.
Thermocouple connections
A thermocouple consists of two dissimilar metals that are joined together at one end. When the junction of the
metals is heated or cooled, a voltage is produced that correlates to temperature.
The OM-USB-TEMP-AI makes fully differential thermocouple measurements without requiring ground-
referencing resistors. A 32-bit floating point value in either a voltage or temperature format is returned by
software. An open thermocouple detection (OTD) feature is available for each thermocouple input. This feature
automatically detects an open or broken thermocouple.
Use InstaCal to select the thermocouple type (J, K, R, S, T, N, E, and B) on one or more sensor input channels
to connect the thermocouple.
Wiring configuration
Connect the thermocouple to the OM-USB-TEMP-AI using a differential configuration, as shown in Figure 3.
Figure 3. Typical thermocouple connection
The OM-USB-TEMP-AI GND pins are isolated from earth ground. You can connect thermocouple sensors to
voltages referenced to earth ground as long as the isolation between the GND pins and earth ground is
maintained.
When thermocouples are attached to conductive surfaces, the voltage differential between multiple
thermocouples must remain within ±1.4 V. For best results, we recommend the use of insulated or ungrounded
thermocouples when possible.
Maximum input voltage between analog input and ground
The absolute maximum input voltage between an analog input and the isolated GND pins is ±25 VDC when the
OM-USB-TEMP-AI is powered on, and ±40 VDC when the OM-USB-TEMP-AI is powered off.
If you need to increase the length of your thermocouple, use the same type of thermocouple wires to minimize
the error introduced by thermal EMFs.
RTD and thermistor connections
A resistance temperature detector (RTD) measures temperature by correlating the resistance of the RTD
element with temperature. A thermistor is a thermally-sensitive resistor that is similar to an RTD in that its
resistance changes with temperature — thermistors show a large change in resistance that is proportional to a
small change in temperature. The main difference between RTD and thermistor measurements is the method
used to linearize the sensor data.
RTDs and thermistors are resistive devices that require an excitation current to produce a voltage drop that can
be measured differentially across the sensor. The OM-USB-TEMP-AI features two built-in current excitation
sources (±I1 and ±I2) for measuring resistive type sensors. Each current excitation terminal is dedicated to one
channel pair.

OM-USB-TEMP-AI User's Guide Signal I/O Connections
15
The OM-USB-TEMP-AI makes two, three, and four-wire measurements of RTDs (100 _ platinum type) and
thermistors.
Use InstaCal to select the sensor type and the wiring configuration. Once the resistance value is calculated, the
value is linearized in order to convert it to a temperature value. A 32-bit floating point value in either
temperature or resistance is returned by software.
RTD maximum resistance
Resistance values greater than 660 _ cannot be measured by the OM-USB-TEMP-AI in the RTD mode. The
660 _ resistance limit includes the total resistance across the current excitation (±Ix) pins, which is the sum of
the RTD resistance and the lead resistances.
Thermistor maximum resistance
Resistance values greater than 180 k_ cannot be measured by the OM-USB-TEMP-AI in the thermistor mode.
The 180 k_ resistance limit includes the total resistance across the current excitation (±Ix) pins, which is the
sum of the thermistor resistance and the lead resistance.
Two-wire configuration
The easiest way to connect an RTD sensor or thermistor to the OM-USB-TEMP-AI is with a two-wire
configuration, since it requires the fewest connections to the sensor. With this method, the two wires that
provide the RTD sensor with its excitation current also measure the voltage across the sensor.
Since RTDs exhibit a low nominal resistance, measurement accuracy can be affected due to the lead wire
resistance. For example, connecting lead wires that have a resistance of 1 _ (0.5 _ each lead) to a 100 _ platinum
RTD will result in a 1% measurement error.
With a two-wire configuration, you can connect either one sensor per channel pair, or two sensors per channel
pair.
Two-wire, single-sensor
A two-wire single-sensor measurement configuration is shown in Figure 4.
Figure 4. Two-wire, single RTD or thermistor sensor measurement configuration
When you select a two-wire single sensor configuration with InstaCal, connections to T#H and T#L are made
internally.
Two-wire, two sensor
A two-wire, two-sensor measurement configuration is shown in Figure 5.

OM-USB-TEMP-AI User's Guide Signal I/O Connections
16
Figure 5. Two-wire, two RTD or thermistor sensors measurement configuration
When you select a two-wire, two sensor configuration with InstaCal, connections to T#H (first sensor) and
T#H/T#L (second sensor) are made internally.
When configured for two-wire mode, both sensors must be connected to obtain proper measurements.
Three-wire configuration
A three-wire configuration compensates for lead-wire resistance by using a single voltage sense connection.
With a three-wire configuration, you can connect only one sensor per channel pair. A three-wire measurement
configuration is shown in Figure 6.
Figure 6. Three-wire RTD or thermistor sensor measurement configuration
When you select a three-wire sensor configuration with InstaCal, the OM-USB-TEMP-AI measures the lead
resistance on the first channel (T#H/T#L) and measures the sensor itself using the second channel (T#H/T#L).
This configuration compensates for any lead-wire resistance and temperature change in lead-wire resistance.
Connections to T#H for the first channel and T#H/T#L of the second channel are made internally.
Three-wire compensation
For accurate three wire compensation, the individual lead resistances connected to the ±I# pins must be of equal
resistance value.
Four-wire configuration
With a four-wire configuration, connect two sets of sense/excitation wires at each end of the RTD or thermistor
sensor. This configuration completely compensates for any lead-wire resistance and temperature change in lead-
wire resistance.
Connect your sensor with a four-wire configuration when your application requires very high accuracy
measurements. Examples of a four-wire single-sensor measurement configuration are shown in Figure 7 and
Figure 8.
You can configure the OM-USB-TEMP-AI with either a single sensor per channel or two sensors per channel
pair.
Four-wire, single-sensor
A four-wire, single-sensor connected to the first channel of a channel pair is shown in Figure 7.

OM-USB-TEMP-AI User's Guide Signal I/O Connections
17
Figure 7. Four-wire, single RTD or thermistor sensor measurement configuration
A four-wire, single-sensor connected to the second channel of a channel pair is shown in Figure 8.
Figure 8. Four-wire, single RTD or thermistor sensor measurement configuration
A four-wire, two-sensor measurement configuration is shown in Figure 9.
Figure 9. Four-wire, two RTD or thermistor sensors measurement configuration
When configured for four-wire, two sensor mode, both sensors must be connected to obtain proper
measurements.
Semiconductor sensor measurements
Semiconductor sensors are suitable over a range of approximately -40 °C to 125 °C, where an accuracy of
±2 °C is adequate. The temperature measurement range of a semiconductor sensor is small when compared to
thermocouples and RTDs. However, semiconductor sensors can be accurate, inexpensive and easy to interface
with other electronics for display and control.
The OM-USB-TEMP-AI makes high-resolution measurements of semiconductor sensors, such as the LM36 or
equivalent, and returns a 32-bit floating point value in either a voltage or temperature format.
Use InstaCal to select the sensor type (TMP36 or equivalent) and the sensor input channel to connect the
sensor.

OM-USB-TEMP-AI User's Guide Signal I/O Connections
18
Wiring configuration
You can connect a TMP36 (or equivalent) semiconductor sensor using a single-ended configuration, as shown
in Figure 10. The OM-USB-TEMP-AI also provides +5V and GND pins for powering the sensor.
Figure 10. Semiconductor sensor measurement configuration
The software outputs the measurement data as a 32-bit floating point value in either voltage or temperature.
Digital I/O connections
You can connect up to eight digital I/O lines to the screw terminals labeled DIO0 to DIO7. You can configure
each digital bit for either input or output. All digital I/O lines are pulled up to +5V with a 47 k_ resistor
(default). You can request the factory to configure the resistor for pull-down to ground if desired.
When you configure the digital bits for input, you can use the OM-USB-TEMP-AI digital I/O terminals to
detect the state of a TTL-compatible device. Refer to the schematic shown in Figure 11. If you set the switch to
the +5V input, DIO0 reads TRUE (1). If you move the switch to GND, DIO0 reads FALSE (0).
Figure 11. Schematic showing switch detection by digital channel DIO0
Caution! All ground pins on the OM-USB-TEMP-AI (pins 9, 19, 22, 27, 30, 33, 36, 39, 49) are common
and are isolated from earth ground. If a connection is made to earth ground when using digital I/O
and conductive thermocouples, the thermocouples are no longer isolated. In this case,
thermocouples must not be connected to any conductive surfaces that may be referenced to
earth ground.
For general information regarding digital signal connections and digital I/O techniques, refer to the General
Guide to Signal Connections (available on our web site at www.omega.com/manuals/manualpdf/M4830.pdf).


Chapter 4
Functional Details
Thermocouple measurements
A thermocouple consists of two dissimilar metals that are joined together at one end. When the junction of the
metals is heated or cooled, a voltage is produced that correlates to temperature.
The OM-USB-TEMP-AI hardware level-shifts the thermocouple’s output voltage into the A/D’s common mode
input range by applying +2.5 V to the thermocouple’s low side at the C#L input. Always connect thermocouple
sensors to the OM-USB-TEMP-AI in a floating fashion. Do not attempt to connect the thermocouple low side
C#L to GND or to a ground referencing resistor.
Cold junction compensation (CJC)
When you connect the thermocouple sensor leads to the sensor input channel, the dissimilar metals at the OM-
USB-TEMP-AI terminal blocks produce two additional thermocouple junctions. This junction creates a small
voltage error term which must be removed from the overall sensor measurement using a cold junction
compensation technique. The measured voltage includes both the thermocouple voltage and the cold junction
voltage. To compensate for the additional cold junction voltage, the OM-USB-TEMP-AI subtracts the cold
junction voltage from the thermocouple voltage.
The OM-USB-TEMP-AI has one high-resolution temperature sensor integrated into the design. The CJC sensor
measures the average temperature at the terminal block so that the cold junction voltage can be calculated. A
software algorithm automatically corrects for the additional thermocouples created at the terminal blocks by
subtracting the calculated cold junction voltage from the analog input's thermocouple voltage measurement.
Increasing the thermocouple length
If you need to increase the length of your thermocouple, use the same type of thermocouple wires to minimize
the error introduced by thermal EMFs.
Data linearization
After the CJC correction is performed on the measurement data, an on-board microcontroller automatically
linearizes the thermocouple measurement data using National Institute of Standards and Technology (NIST)
linearization coefficients for the selected thermocouple type. The measurement data is then output as a 32-bit
floating point value in the configured format (voltage or temperature).
Open-thermocouple detection (OTD)
The OM-USB-TEMP-AI is equipped with open-thermocouple detection for each analog input channel. With
OTD, any open-circuit or short-circuit condition at the thermocouple sensor is detected by the software. An
open channel is detected by driving the input voltage to a negative value outside the range of any thermocouple
output. The software recognizes this as an invalid reading and flags the appropriate channel. The software
continues to sample all channels when OTD is detected.
RTD and thermistor measurements
RTDs and thermistors are resistive devices that require an excitation current to produce a voltage drop that can
be measured differentially across the sensor. The OM-USB-TEMP-AI measures the sensor resistance by forcing
a known excitation current through the sensor and then measuring (differentially) the voltage across the sensor
to determine its resistance.
After the voltage measurement is made, the resistance of the RTD is calculated using Ohms law – the sensor
resistance is calculated by dividing the measured voltage by the current excitation level (±Ix) source. The value
of the ±Ix source is stored in local memory.

OM-USB-TEMP-AI User's Guide Functional Details
20
Once the resistance value is calculated, the value is linearized in order to convert it to a temperature value. The
measurement is returned by software as a 32-bit floating point value in a voltage, resistance or temperature
format.
Data linearization
An on-board microcontroller automatically performs linearization on RTD and thermistor measurements.
RTD measurements are linearized using a Callendar-Van Dusen coefficients algorithm (you select DIN,
SAMA, or ITS-90).
Thermistor measurements are linearized using a Steinhart-Hart linearization algorithm (you supply the
coefficients from the sensor manufacturer's data sheet).
USB connector
The USB connector provides +5V power and communication. No external power supply is required.
LED
The LED indicates the communication status of the OM-USB-TEMP-AI. It uses up to 5 mA of current. The
table below defines the function of the OM-USB-TEMP-AI LED.
LED Illumination
LED
Illumination Indication
Steady green The OM-USB-TEMP-AI is connected to a computer or external USB hub.
Pulsing green Data is being transferred.
Upon connection, the LED should flash three times and then remain lit (indicates a successful
installation).
Power
The +5V terminal is isolated (500 VDC) from the USB +5V.
Caution! The +5V terminal is an output terminal. Do not connect to an external power supply or you may
damage the OM-USB-TEMP-AI and possibly the computer.